An anterior floor-reaction orthosis (AFRO) is a brace that prevents the ankle joint from dorsiflexing. In the case of a paralysed limb, without any brace, the ground reaction vector passes through the ankle joint because no torque can be transmitted. However, with an AFRO, the base of the ground reaction vector can be moved forward to the metatarsal region of the foot as shown in FIG. 1a. In this position the ground reaction vector acts ahead of the anatomical knee joint axis and thus stabilises the knee in extension without muscular action.
Traditionally, a paralysed leg is braced using a knee-ankle-foot orthosis (KAFO) that incorporates a mechanically lockable joint positioned at the side of the anatomical knee. In contrast, the anterior floor-reaction orthosis (AFRO) principle may be used to stabilise the paralysed leg without physically locking the knee in extension. This has the major advantage that during the swing phase of walking the knee can flex to gain ground clearance. In addition, this provides improved cosmesis and reduces weight and cumbrance. However, the AFRO only stabilises the knee on the condition that the ground reaction vector is ahead of the knee axis as shown in FIG. 1a. If the ground reaction vector should shift behind the knee as shown in FIG. 1b, then the knee would buckle and the leg would collapse if there were no knee extensor muscle action. Thus, the AFRO is only used or prescribed when the patient has sufficient control of his knee extensor musculature to avoid collapse in these situations. Alternatively, paralysed muscles may be electrically activated in response to such an incident as disclosed in U.S. Pat. No. 5,121,747.